Process for forming a microcrystalline silicon series thin film and apparatus suitable for practicing said process

ABSTRACT

A process for forming a microcrystalline silicon series thin film by arranging a long substrate in a vacuum chamber so as to oppose an electrode provided in said vacuum chamber and while transporting said long substrate in a longitudinal direction, causing glow discharge between said electrode and said long substrate to deposit said microcrystalline silicon series thin film on said long substrate, wherein a plurality of bar-like shaped electrodes as said electrode are arranged such that they are perpendicular to a normal line of said long substrate and their intervals to said long substrate are all or partially differed, and said glow discharge is caused using a high frequency power with an oscillation frequency in a range of from 50 MHz to 550 MHz, whereby depositing said microcrystalline series thin film on said long substrate. An apparatus suitable for practicing said process.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates a process for forming amicrocrystalline silicon series thin film (this film will be hereinafterreferred to as “μc-siicon series thin film” or “μc-Si series thin film”)and an apparatus suitable for practicing said process. Moreparticularly, the present invention relates a process and an apparatuswhich enable one to form a highly reliable μc-Si series thin film havinga large area and a high energy conversion efficiency which is usable inthe production of semiconductor devices such as electrophotographiclight receiving members (or electrophotographic photosensitive members),image input line sensors, image pickup devices, photovoltaic devices(including solar cells), and the like.

[0003] 2. Related Background Art

[0004] Hitherto, solar cells comprising a photovoltaic element whichconverts sunlight into electric energy have been widely using as a smallpower source in daily appliances such as electronic calculators, wristwatches, and the like. And the technique of such solar cell is expectedto provide a practically usable power generation source which canreplace the power generation source based on fossil fuel such aspetroleum.

[0005] Incidentally, a solar cell is based on a technique in whichphtoelectromotive force of a pn junction is used in the functionalportion. In general, the pn junction is constituted by a semiconductormaterial such as a semiconductor silicon material or a semiconductorgermanium material. The semiconductor functions to absorb sunlight andgenerate photocarriers of electron and hole, where the photocarriers aredrifted by an internal electric field of the pn junction, followed bybeing outputted to the outside.

[0006] Now, in view of the efficiency of converting light energy into anelectricity, it is preferred to use a single crystalline siliconmaterial. However, crystalline silicon materials including a singlecrystalline silicon material have an indirect optical end, andtherefore, they are small in light absorption. In this connection, inthe case of a solar cell in which a single crystalline silicon is used(this solar cell will be hereinafter referred to as “single crystalsolar cell”), it is necessary for the single crystal solar cell to havea thickness of at least 50 μm in order for the solar cell tosufficiently absorb incident sunlight. In this case, if the singlecrystalline silicon material should be replaced by a polycrystallinesilicon material in order to diminish the production cost of the solarcell, the problem of the above indirect optical end cannot be solvedunless the thickness is thickened. For the polycrystalline siliconmaterial, it has problems such as grain boundaries and others.

[0007] In view of attaining a large area and a reasonable productioncost for a solar cell, a so-called thin film silicon solar cell which isrepresented by an amorphous silicon solar cell having a semiconductorlayer comprising an amorphous silicon thin film produced by way of CVD(chemical vapor phase deposition) has been evaluated as being moreadvantageous. In fact, currently, amorphous silicon solar cells havebeen widely using as a small power source in daily appliances. However,in order for such amorphous silicon solar cell to be used as an ordinarypower generation source, there is still a subject to more improve thephotoelectric conversion efficiency and to more stabilize theperformance.

[0008] By the way, there has been proposed a solar cell in which amicrocrystalline silicon (a μc-silicon) as a carrier generation layer(see, A. Shah et al., 23th IEEE Photovoltaic Specialist Conf. (1993), p.839) [This document will be hereinafter referred to as document A].

[0009] The most popularized film-forming method for depositing suchμc-siicon series thin film or amorphous silicon thin film is a plasmaCVD process. In the plasma CVD process, the formation of a μc-siiconseries thin film or a amorphous silicon thin film is conducted, forinstance, in the following manner. That is, a film-forming raw materialgas such as silane (SiH₄) or disilane (Si₂H₆) is introduced into areaction chamber in which a substrate on which a film is to be depositedis arranged, if necessary, while being diluted by hydrogen gas (H₂), ahigh frequency power with an oscillation frequency of 13.56 MHz in an RFband region is supplied in the reaction chamber to generate plasmawhereby decomposing the film-forming raw material gas to produce activespecies having reactivity, resulting in depositing a μc-siicon thin filmor a amorphous silicon thin film on the substrate. In the case where thefilm formation is conducted by mixing a doping gas such as phosphine(PH₃), diborane (B₂H₆) or boron fluoride (BF₃) to the film-forming rawmaterial gas, it is possible to form a doped μc-siicon thin film whoseconductivity being controlled to n-type or p-type.

[0010] However, such μc-siicon thin film has a disadvantage such thatthe photoelectric conversion efficiency of a solar cell in which suchμc-siicon thin film is used is lower than that of a crystalline seriessolar cell. In addition, for the μc-siicon thin film, there is also adisadvantage in that the deposition rate upon the formation thereof islow.

[0011] In general, the formation of a μc-siicon thin film is conductedby using RF glow discharge. However, the μc-siicon thin film thus formedhas an indirect optical end as well as in the case of a crystallinesilicon thin film, and therefore, its light absorption is small. In thisconnection, in the case of a μc-silicon solar cell in which a μc-siiconthin film is used, it is necessary for the μc-silicon solar cell to havea thickness of about 5 μm, and therefore, a lot of time is required toproduce the μc-silicon solar cell.

[0012] Additionally saying, in the above mentioned document A, there isdescribed that the formation of a μc-siicon thin film is conducted usinga high frequency power with an oscillation frequency of 70 MHz. It canbe said that the deposition rate in this case is about 1 Å/sec. which ismore small.

[0013] With respect to the formation of an amorphous silicon (a-Si) thinfilm by way of RF plasma CVD, there is a report in that for the highfrequency discharge in the RF band region hitherto, discussion has beenmade by raising the oscillation frequency. Particularly, in appliedphysics-related joint lecture meetings of 1990 Autumn and 1991 Spring(28p-MF-14 and 28p-S-4), Oda et als. of Tokyo Institute of Technologyhave reported that amorphous silicon thin films were formed byconducting glow discharge using a high frequency power with anoscillation frequency of 144 MHz (which is of VHF (very high frequency)band region) and the amorphous silicon thin films were evaluated. [These28p-MF-14 and 28p-S-4 will be hereinafter referred to as document B.]

[0014] Besides, U.S. Pat. No. 4,400,409 discloses a process ofcontinuously preparing a photovoltaic element by using a continuousplasma CVD apparatus of a roll-to-roll system. In this document, thereis described that a plurality of glow discharge regions are separatelyarranged along the path of a sufficiently long flexible substrate havinga desired width which is continuously transported which passing througheach of said glow discharge regions, and while forming a desiredsemiconductor layer on the substrate in each glow discharge region, thesubstrate is continuously transported, whereby a photovoltaic elementhaving a desired semiconductor junction can be continuously formed.

[0015] By the way, in the case of forming a μc-siicon series thin filmby means RF glow discharge using a high frequency power with anoscillation frequency of 13.56 MHz as in the foregoing prior art, thereare such subjects as will be described in the following which arenecessary to be solved or improved.

[0016] (1) For semiconductor devices in which such μc-siicon thin film,because of the basic property of the thin film, there are suchdisadvantages as will be described in the following. That is, in thecase of a thin film transistor, the carrier mobility is small. In thecase of a photo sensor, its S/N ration defined by a ratio between lightconductivity and that dark conductivity. In the case of a solar cell,its photoconductivity (σp) is small.

[0017] (2) With respect to production yield, in the case of a large areasemiconductor device in which such μc-siicon series thin film is used, adecrease in the yield is caused due to the distributions and the like ofdevice characteristics which are based on the distributions of filmthickness and film quality.

[0018] (3) with respect to production cost, in the case of forming ahigh quality μc-siicon thin film usable in a thin film semiconductordevice, the productivity cannot be increased because the deposition rateis small, resulting in an increase in the production cost.

[0019] (4) With respect to property, in the formation of such μc-siiconthin film, it is difficult for the μc-siicon thin film to have a desiredproperty controlled in the film thickness direction.

[0020] Eventually, in order to produce a large area μc-siicon thin filmsolar cell having improved device characteristics at a high yield and ata reasonable production cost, it is necessary to form a μc-siicon thinfilm at a high deposition rate while improving the basic propertythereof. In addition, it is necessary to realize a method which enablesto readily control the property in the film thickness direction.

[0021] In order to attain this object, in the plasma CVD process of13.56 MHz, improvements in the production conditions such as flow rateof raw material gas, pressure upon film formation, power applied and thelike have been generally tried. However, in any case, it is liable tooccur such problems as will be described in the following. That is, whenthe deposition rate is increased, a film deposited becomes to have anamorphous nature (that is, the film becomes to be an a-Si film), theamount of in-film hydrogen which is presumed to deteriorate the propertyof a μc-siicon thin film is increased, or foreign matter which cases areduction in the yield is generated. Specifically, for instance, as thedeposition rate is increased, the photoconductivity σp as the basicproperty of the μc-siicon thin film is decreased. In this connection, inthis process of forming a μc-siicon thin film, the deposition ratecapable of maintaining desirable device characteristics is in a range offrom about 0.2 to about 2 Å/sec.

[0022] In the RF glow discharge process, the range for controllableparameters capable of forming a μc-siicon thin film having a goodquality is narrow, where it is difficult to control the property of theμc-siicon thin film as desired.

[0023] The RF discharge process of 13.6 MHz has an advantage in thatfilm formation on a large area can be readily conducted. However, it hasdisadvantages such that the deposition rate is small and ion damage to asubstrate or a μc-siicon thin film itself deposited thereon is large. Inthis connection, there is occasionally used a triode process in which athird electrode is provided between an anode and a cathode. However,this process is not suitable in terms of industrial production of aμc-siicon thin film, because the utilization efficiency of raw materialgas is undesirably small and the maintenance efficiency is notsatisfactory. In this respect, this triode process is used only forresearch purposes. In addition, in the case of forming a μc-siicon thinfilm by the triode process, it is difficult to control the propertythereof as desired. Besides, in the case of forming a μc-siicon thinfilm by means of a microwave discharge process of 2.54 GHz, althoughthere are advantages such that the deposition rate is relatively largeand the ion damage to the substrate is not occurred, there aredisadvantages such that it is difficult to continuously maintain theglow discharge according to the current technique, and the processcontrollability is not good. In addition, there are other disadvantagesuch that gas decomposition at a microwave introduction portion is greatand therefore, it is difficult to conduct uniform deposition. In thecase of a photo CVD process, there are disadvantages such that thequality of a μc-siicon thin film deposited is not good and it isdifficult to deposit a μc-siicon thin film on a large area. In addition,the photo CVD process is a film-forming technique on the way todevelopment. For an ECR-CVD process, since it is possible to freelycontrol the ion damage to the substrate, there is a possibility offorming a high quality μc-siicon thin film. But because of usingmagnetic field, it is difficult to deposit such μc-siicon thin film inan essentially uniform state.

[0024] As above described, it is difficult to effectively produce a highμc-siicon thin film semiconductor device at good reproducibility by anyof the conventional techniques, because for a μc-siicon thin filmformed, it is difficult make the μc-siicon thin film have a high qualityso as to enable to effectively produce a desirable semiconductor devicehaving satisfactory device characteristics; it is difficult to make theμc-siicon thin film have a property controlled in the film thicknessdirection; and it is difficult to stably and repeatedly form a highquality μc-siicon thin film having a desired property at a highdeposition rate and at good reproducibility. In addition to these, theconditions for causing microcrstallization in the prior art are severe,and therefore, it is difficult to stably and repeatedly form a desiredμc-siicon thin film.

[0025] By the way, in the foregoing document B and Japanese UnexaminedPatent Publication No. 64466/1991 which is of the technique similar tothat of document B, discussion is made only of amorphous silicon (a-Si)thin film but no discussion is made of microcrystalline silicon (μc-Si)thin films. In the foregoing document A, there is no touch on optimumconditions for the formation of a μc-Si) thin film and no discussion ismade about the control of the property in the film thickness direction.The technique described in this document is not effective in solving thesubjects which the present invention is intended to solve.

SUMMARY OF THE INVENTION

[0026] A principal object of the present invention is to eliminate theforegoing problems in the prior art and to provide an improved processand apparatus which enable to readily and efficiently form a highquality microcrystalline silicon series (μc-Si series) thin film havinga desired property.

[0027] Another object of the present invention is to provide a processand apparatus which enable to readily and efficiently form a μc-Siseries thin film having a desired property capable of producing a highquality semiconductor device, at an improved film-forming raw materialgas utilization efficiency and at a reasonable production cost.

[0028] A further object of the present invention is provide a processfor forming a μc-Si series thin film film in which the property thereofin the film thickness direction can be readily controlled whilemaintaining the film property, where that the problems in the prior artare desirably solved, so that a high quality μc-Si series thin filmhaving a graded film property in the film thickness direction.

[0029] A further object of the present invention is to provide a processand apparatus which enable to produce a high quality μc-Si series thinfilm semiconductor device at a reasonable production cost.

[0030] A further object of the present invention is to provide a processfor forming a μc-Si series thin film by arranging a long substrate in avacuum chamber so as to oppose an electrode provided in said vacuumchamber and while transporting said long substrate in the longitudinaldirection, causing glow discharge between said electrode and thesubstrate to deposit said μc-Si series thin film on the substrate,wherein a plurality of bar-like shaped electrodes as said electrode arearranged such that they are perpendicular to a normal line of said longsubstrate and their intervals to said long substrate are all orpartially differed, and said glow discharge is caused using a highfrequency power with an oscillation frequency in a range of from 50 MHzto 550 MHz, whereby depositing said μc-Si series thin film on thesubstrate.

[0031] A further object of the present invention is to provide anapparatus capable forming a μc-Si series thin film on a long substrate,having a portion in which said long substrate is arranged to oppose toan electrode in a vacuum chamber, wherein while transporting said longsubstrate in the longitudinal direction, glow discharge is causedbetween the electrode and the substrate to deposit said μc-Si seriesthin film on the substrate, wherein said apparatus has a plurality ofbar-like shaped electrodes as said electrode which are arranged suchthat they are perpendicular to a normal line of said long substrate andtheir intervals to said long substrate are all or partially differed anda high frequency power source for causing said glow discharge using ahigh frequency power with an oscillation frequency in a range of from 50MHz to 550 MHz.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a schematic diagram illustrating an example of afilm-forming apparatus of a roll-to-roll system.

[0033]FIG. 2 is a view showing an example of distribution of absorptioncoefficient in a width direction.

[0034]FIG. 3 is a view showing an example of distribution of averagegrain size in a width direction.

[0035]FIG. 4 is a view showing an example of distribution of crystalvolume fraction in a width direction.

[0036]FIG. 5 is a view showing an example of distribution of hydrogencontent in a width direction.

[0037]FIG. 6 is a graph showing an example of relationship betweenabsorption coefficient to light having a wavelength of 800 nm and aninterval between a substrate and an electrode.

[0038]FIG. 7 is a graph showing an example of relationship betweenaverage crystal grain size and an interval between a substrate and anelectrode.

[0039]FIG. 8 is a graph showing an example of relationship betweencrystal volume fraction and an interval between a substrate and anelectrode.

[0040]FIG. 9 is a graph showing an example of relationship betweenhydrogen content and an interval between a substrate and an electrode.

[0041]FIG. 10 is a graph showing an example of relationship between athickness of an amorphous layer formed and an interval between asubstrate and an electrode.

[0042]FIGS. 11, 12, 13, 16, 17, 18 and 20 are schematic cross-sectionalslant views respectively for explaining an example of arrangingrelationship of a substrate and an electrode.

[0043] FIGS. 14(a) and 14(b) are schematic cross-sectional viewsrespectively for explaining an example of a cross section of a μc-Siseries thin film.

[0044]FIGS. 15, 19 and 21 are graphs respectively for explaining anexample of hydrogen content in a depth (thickness) direction of andeposited film.

DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

[0045] The present invention attains the foregoing objects. The presentinvention provides

[0046] The present invention provides a process and apparatus whichenable to readily and efficiently form a μc-Si series thin film having adesired property capable of producing a high quality semiconductordevice, at an improved film-forming raw material gas utilizationefficiency and at a reasonable production cost.

[0047] As above described, a typical embodiment of the process is aprocess for forming a μc-Si series thin film by arranging a longsubstrate in a vacuum chamber so as to oppose an electrode provided insaid vacuum chamber and while transporting said long substrate in thelongitudinal direction, causing glow discharge between said electrodeand the substrate to deposit said μc-Si series thin film on thesubstrate, wherein a plurality of bar-like shaped electrodes as saidelectrode are arranged such that they are perpendicular to a normal lineof said long substrate and their intervals to said long substrate areall or partially differed, and said glow discharge is caused using ahigh frequency power with an oscillation frequency in a range of from 50MHz to 550 MHz, whereby depositing said μc-Si series thin film on thesubstrate.

[0048] And a typical embodiment of the apparatus of the presentinvention is an apparatus for forming a μc-Si series thin film on a longsubstrate, having a portion in which said long substrate is arranged tooppose to an electrode in a vacuum chamber, wherein while transportingsaid long substrate in the longitudinal direction, glow discharge iscaused between the electrode and the substrate to deposit said μc-Siseries thin film on the substrate, wherein said apparatus has aplurality of bar-like shaped electrodes as said electrode which arearranged such that they are perpendicular to a normal line of said longsubstrate and their intervals to said long substrate are all orpartially differed and a high frequency power source for causing saidglow discharge using a high frequency power with an oscillationfrequency in a range of from 50 MHz to 550 MHz.

[0049] The above-described process according to the present inventionincludes the following embodiments with respect to the arrangement ofthe bar-like shaped electrodes.

[0050] (i) The bar-like shaped electrodes are arranged such that theyare in parallel to each other.

[0051] (ii) The bar-like shaped electrodes are arranged such that theyare perpendicular to the transportation direction of the long substrate.

[0052] (iii) The bar-like shaped electrodes are arranged such that theirintervals to the long substrate are widened in the upper side of thetransportation direction of the long substrate and narrowed in the downside thereof.

[0053] (iv) The bar-like shaped electrodes are arranged such that theirintervals to the long substrate are periodically changed to thetransportation direction of the long substrate.

[0054] In the following, description will be made of the constitutionand action of the present invention.

[0055] In the present invention, because of using a frequency band in arange of from 50 MHz to 550 MHz which is between RF (radio frequency)and MW (microwave), it is possible to form a high quality μc-Si seriesthin film at an increased deposition rate, specifically, of more than 10Å/sec.

[0056] Particularly, the frequency in this frequency band is higher thanRF, and because of this, the self bias of plasma is decreased todiminish ion damage to a film deposited on a substrate. Further, incomparison with the MW plasma CVD process, the introduction of a poweris easier and the controllable range is widened, where it is possible toform a high quality thin film. In addition, as a result of experimentalstudies by the present inventors, there was obtained a finding. That is,when film deposition on a long substrate is conducted using a highfrequency power of said frequency band, in the case of a low powerdensity (that is, in a region where the deposition rate is increased inproportion to a power), the property of a film deposited differsdepending on a difference in the interval between the electrode (thecounter electrode) and the long substrate.

[0057] In the case of the above frequency band, in comparison with RF,the maintenance of discharge under low pressure is easier, the electrontemperature is lower, and the electron density is higher, and because ofthis, it is advantageous for the generation of long life radicals(SiH₃*, H*). At the same electron density, in the case of RF, the vaporphase reaction proceeds to produce polysilane. However, in the case ofthe above reference band, even under such condition of producingpolysilane in the case of RF, a desirable deposited film can be formedwithout producing such polysilane. In addition, from the observation ofthe discharge, there is substantially not recognized a positive columnwhich is observed in the plasma in the case of RF but emission isobserved in the vicinity of the cathode. In this connection, it isconsidered such that dissociation is proceeded in the vicinity of thecathode. For the radicals generated in the vicinity of the cathode, itis considered such that they are isotropically diffused, and during thediffusion, in a process of colliding to the parent molecules (SiH₄, H₂),the ion species and the short life radicals contained therein aredisappeared. And it is considered such that depending on a difference inthe interval between the substrate and the electrode, the density andcomponent of the radicals reached to the substrate are changed, and thissituation results in a difference in the property of a film deposited.

[0058] Especially, in the case of a microcrystalline silicon (μc-Si), H*plays an important role for relaxation and rearrangement of the lattice,and a microcrystal is grown by way of combination of this radial with astable site by SiH₃* which is likely to be surface-diffused. Hence, theratio of these radicals and the like are important factors.

[0059] Therefore, in the case of the above-described frequency band, themagnitude of a power applied, the pressure, and the interval between thesubstrate and the electrode are important parameters in order to controlthe property of a film deposited. By properly controlling theseparameters, the grain size, crystal deposition rate, hydrogen content,stress, absorption coefficient and the like can be changed. Further, fora compound such as SiGe:H or SiC:H, it is possible for the compositionratio thereof to be controlled based on a difference in thedecomposition energy or the lifetime of a radical.

[0060] Further in the case of the above-described frequency band, thewavelength is shortened to be equal to the electrode length, when theelectrode comprises a flat electrode, standing wave is present in thein-plane, where a node in which the power is weak and an antinode inwhich the power is strong are occurred. And the positions of these nodeand antinode are different depending on the form of the electrode, thepower introduction position, the state of plasma generated, or the like,and because of this, it is difficult for them to be controlled asdesired in the case of the plat electrode.

[0061] Occurrence of this problem can be prevented by making the platelectrode to comprise a plurality of bar-like shaped electrodes.

[0062] Based on the above-described factors, in the roll-to-rollfilm-forming system, by providing a providing a plurality of bar-likeshaped electrodes and properly adjusting their intervals to a longsubstrate which is continuously transported, the property of amicrocrystal grown on said substrate in the thickness direction can bedesirably controlled.

[0063] In the present invention, by arranging the bar-like shapedelectrodes in parallel to each other, the way of the power introduced byeach electrode in a width direction to the transportation direction ofthe long substrate is equalized and as a result, occurrence of filmunevenness is further diminished. Further, by arranging the bar-likeshaped electrodes to be perpendicular to the transportation direction ofthe long substrate, the film deposition position by each electrode isequalized to the transportation distance of the long substrate and as aresult, occurrence of film unevenness due to the form of a vacuumchamber used or the passage of gas is further diminished.

[0064] As above described, the interval between the substrate and theelectrode influences to the property of a film deposited. In view of asemiconductor device to be produced, a μc-Si series thin film isdeposited on an amorphous substrate comprising glass or the like or on apolycrystalline substrate comprising TCO (transparent electricallyconductive oxide) or the like. In this case, there are such occasionsthat in the vicinity of the interface between the substrate and theμc-Si series thin film, an amorphous material of several tens to severalhundreds nm is present or a layer of a small grain size is present. Inmany cases, being different from an amorphous silicon of a device grade,such layer is inferior in quality. Particularly, in the case of asemiconductor device such as a solar cell in which an electron islongitudinally mobilized, such layer is liable to deteriorate itselectric characteristics. Therefore, it is desired to make such that nosuch layer is present or when it should be present, its thickness isthinned.

[0065] Now, when a microcrystal having a large grain size and goodquality is intended to deposit from a initial deposition stage, thefollowing are important factors.

[0066] (1) Generation of a homogeneous crystalline nucleus of lowdensity.

[0067] (2) Increase in crystal grain size.

[0068] (3) High speed growth-in film thickness direction.

[0069] For the factor (1), when a highly dense crystalline nucleus isgenerated, though it is considered that coalescing of grains would beoccurred, the grain size is substantially limited to an area occupied byone crystalline nucleus. Therefore, in the crystalline nucleusgeneration process, it is desired to employ condition capable ofgenerating a crystalline nucleus of low density without causingdeposition of an amorphous layer.

[0070] For the factor (2), ideally, it is desired to employ suchcondition that the crystalline nucleus is crosswise grown withoutcausing a new crystalline nucleus. By this, there can be obtained alarge grain size from the initial deposition stage.

[0071] For the factor (3), in the case of using a microcrystal in asemiconductor device, a thickness of about 1 μm is necessitated. Inorder to shorten the time required in the preparation of thesemiconductor device, a high speed deposition rate is necessitated. On alayer comprising a crystal of a μc-Si series thin film, crystallinity ofgood quality can be maintained by taking over the crystallinity of theunder layer even when the deposition rate is increased to a certainextent.

[0072] Therefore, to grade the intervals of the bar-like shapedelectrodes to the long substrate such that they are wide in the upperside of the transportation direction of the long substrate which iscorresponding to the initial deposition stage and narrow in the downside is preferred in a viewpoint that the above process can be realized.

[0073] Besides, in the case of a semiconductor device such as a solarcell into which light is impinged from above, when it is constitutedsuch that short wavelength light is absorbed in a region near the lightincident side and long wavelength light is absorbed in a region remotefrom the light incident side, the semiconductor device can effectivelyabsorb light. Therefore, the absorption coefficient is preferred to besuch that it is decreased as the deposition proceeds. In thisconnection, by grading the intervals of the bar-like shaped electrodesto the long substrate such that they are wide in the upper side of thetransportation direction of the long substrate which is corresponding tothe initial deposition stage and narrow in the down side, thepreparation of a semiconductor device having such constitution as abovedescribed can be realized.

[0074] Further, as above described, the property of a film deposited canbe changed by changing the intervals of the electrodes to the substrate.Therefore, by arranging the bar-like shaped electrodes such that theirintervals to the long substrate are periodically changed to thetransportation direction of the long substrate, the property of a filmdeposited can be periodically changed. By this, the absorptioncoefficient is periodically changed. And in the case of forming a solarcell using such deposited film whose absorption coefficient beingperiodically changed, there can be together attained large Voc and Jscwhich are greater than those in the case of using a deposited filmhaving a simple absorption coefficient. In addition, the stress of thefilm is changed so that the film is hardly peeled due a differencebetween the stress of the substrate and that of the film. Further, theresultant becomes desirably endurable upon curving or bendingprocessing.

[0075] In the present invention, a film whose property beingperiodically changed and which has such advantages as above describedcan be readily realized in one deposited film.

[0076] And by using the previously described apparatus, such processesabove described can readily conducted.

[0077] In the following, the process for forming a μc-Si series thinfilm according to the present invention will be detailed.

[0078] μc-Si Series Thin Film:

[0079] Description will be made of a μc-Si series thin film according tothe present invention which can be desirably used in a photovoltaicelement.

[0080] As the constituent of the μc-Si series thin film, there can bementioned materials containing Si element as a matrix. Specific examplesof such material are group IV alloys such as SiGe, SiC, SiSn and thelike.

[0081] As the μc-Si series thin film, there can be used appropriatemicrocrystalline semiconductor materials such as group IVmicrocrystalline semiconductor materials and group IV alloy seriesmicrocrystalline semiconductor materials. Preferable specific examplesare μc-Si:H (hydrogenated microcrystalline silicon), μc-Si:F, μc-Si:H:F,μc-SiGe:H, μc-SiGe:F, μc-SiGe:H:F, μc-SiC:H, μc-SiC:F, and μc-SiC:H:F.

[0082] Incidentally, for a semiconductor layer, valence electron controlor forbidden band width control can be conducted. The control of theseitems can be conducted by separately introducing a raw material compoundcontaining an element capable of being a valence electron controllingagent into a film-forming space upon forming a semiconductor layer or bymixing said raw material compound with a film-forming raw material gasor a dilution gas, followed by introducing into the film-forming space.

[0083] In the present invention, in the case where a μc-Si series thinfilm according to the present invention is subjected to valence electroncontrol by means a valence electron controlling agent, at least partthereof is doped to be p-type and n-type, whereby forming at least a pinconjunction. And by stacking a plurality of pin junctions, there can beformed a stacked cell structure.

[0084] In the case of using a μc-Si series thin film according to thepresent invention as a power generation layer (a semiconductorphotoactive layer) of a photovoltaic element, the average grain size ispreferred to be in a range of from 10 nm to 1 μm, the crystal depositionrate is preferred to be in a range of from 10% to 99%, and the hydrogencontent is preferred to be in a range of from 0.1 atomic % to 40 atomic%.

[0085] Formation of μc-Si Series Thin Film:

[0086] In the process of forming a μc-Si series thin film according tothe present invention, by means of a high frequency power with anoscillation frequency in an range of from 50 MHz to 550 MHz, plasma isgenerated to dissociate and decompose a raw material gas, wherebycausing film deposition on a substrate.

[0087] Specifically, starting gases such as a film-forming raw materialgas, dilution gas and the like are introduced into a deposition chamber(a vacuum reaction chamber) whose inside being capable of maintainingunder reduced pressure, the inner pressure of the deposition chamber ismade to be constant while evacuating the inside of the depositionchamber by means of a vacuum pump, and a high frequency power with adesired oscillation frequency in the foregoing range from a highfrequency power source is supplied to bar-like shaped electrodesarranged in the deposition chamber through a cable and a waveguide togenerate plasma of said gases and decompose said gases, whereby forminga desired μc-Si series thin film on a substrate. According to theprocess of the present invention, it is possible to form a desirableμc-Si series thin film usable in a photovoltaic element under depositioncondition which is relatively wide.

[0088] The substrate temperature in the deposition chamber upon the filmformation is preferred to be in a range of from 100 to 500° C.Similarly, the inner pressure is preferred to be in an range of from1.3×10⁻¹ to 1.3×10² Pa. For the wattage of the high frequency power, itis preferred to be in an range of from 0.05 to 50 W/cm².

[0089] For the bar-like shaped electrodes used in the process of forminga μc-Si series thin film, each of them is such that the length is longerthan a diameter or a longest side of the cross section. Each of thebar-like shaped electrode may be of a cross section in a round form, anellipsoidal form, a square form or a rectangular form. Each of thebar-like shaped electrodes is constituted by a material which is notmelted with heat and which does not cause reaction. Specific examples ofsuch material are stainless steel and the like.

[0090] All the bar-like shaped electrodes are not necessary to beuniform with respect to their forms, but their forms may be varied. Forthe number of the bar-like shaped electrodes, it is properly determineddepending on the size of a vacuum chamber (a deposition chamber) used,the deposition rate and the property for a μc-Si series thin film to beformed.

[0091] As described in Experiments 2 and 3 which will be describedlater, by adjusting the interval between the substrate and theelectrode, the property of a μc-Si series thin film to be formed can bechanged. Therefore, the bar-like shaped electrodes are desired to bearranged such that their intervals to the substrate (the long substrate)are all or partially varied, so that a desirable a μc-Si series thinfilm can be formed. In a more preferred embodiment in this case, theintervals of the bar-like shaped electrodes to the long substrate aregraded such that they are wide in the upper side of the transportationdirection of the long substrate which is corresponding to the initialdeposition stage and narrow in the down side. In order to form adesirable a μc-Si series thin film having different properties, it ispreferred to arrange the bar-like shaped electrodes such that theirintervals to the substrate (the long substrate) are partiallyperiodically changed. [The intervals of the bar-like shaped electrodesto the substrate (the long substrate) will be hereinafter simplyreferred to as “interval between the substrate and the electrode”]

[0092] For the interval between the substrate and the electrode, it isdifferent depending on related film-forming condition for forming adesired μc-Si series thin film. However, in view of a deposition ratewhich can be industrially employed in practice and also in terms ofstability in the glow discharge, it is preferred to be in a range offrom about 0.5 cm to about 30 cm.

[0093] The supply of a high frequency power to each of the bar-likeshaped electrodes may be conducted by a manner of divergently supplyinga high frequency power to each of the bar-like shaped electrodes from asingle high frequency power source. Alternatively, it may be conductedby using a plurality of high frequency power sources. In this case,these high frequency power sources are desired to be complete withrespect to their stability and conformity of oscillation frequency. Ifnecessary, it is possible to take a procedure of conforming theiroscillation frequency by using a phase shifter or the like.

[0094] As the film-forming raw material gas suitable for forming a μc-Siseries thin film of the present invention, there can be representativelymentioned silicon (Si)-containing compounds which are in the gaseousstate at room temperature or can be easily gasified, germanium(Ge)-containing compounds which are in the gaseous state at roomtemperature or can be easily gasified, carbon (C)-containing compoundswhich are in the gaseous state at room temperature or can be easilygasified, and mixtures of these compounds.

[0095] Such Si-containing compound can include chain or cyclic silanecompounds. Specific examples are SiH₄, Si₂H₆, SiF₄, SiFH₃, SiF₂H₂,SiF₃H, Si₃H₈, SiD₄, SiHD₃, SiH₂D₂, SiH₃D, SiFD₃, SiF₂D₂, Si₂D₃H₃,(SiF₂)₅, (SiF₂)₆, (SiF₂),₄, Si₂F₆, Si₃F₈, Si₂H₂F₄, Si₂H₃F₃, (SiCl₂)₅,SiBr₄, (SiBr₂)₅, Si₂Cl₆, SiHCl₃, SiH₂Br₂, SiH₂Cl₂, and Si₂Cl₃F₃.

[0096] Specific examples of such Ge-containing compound are GeH₄, GeD₄,GeF₄, GeFH₃, GeF₂H₂, GeF₃H, GeHD₃, GeH₂D₂, GeH₃D, Ge₃H₆, and Ge₂D₆.

[0097] Specific examples of such C-containing compound are CH₄, CD₄,C_(n)H_(n-2) (with n being an integer), C_(n)H_(2n) (with n being aninteger), C₂H₂, C₆H₆, CO₂, and CO.

[0098] Other than the above-mentioned raw materials, it is possible touse nitrogen (N)-containing raw material gas and oxygen (O)-containingraw material gas. Specific examples of such N-containing raw materialgas are N₂, NH₃, ND₃, NO, NO₂, and N₂O. Specific examples of suchO-containing raw material gas are O₂, CO, CO₂, NO, N₂O, CH₃CH₂OH, andCH₃OH.

[0099] As above described, a μc-Si series thin film of the presentinvention may be controlled so as to have a conductivity of p-type orn-type by incorporating an appropriate valence electron controllingelement (that is, a dopant) thereinto. Such element can include elementsbelonging to group IIIb of the periodic table which provide a p-typeconductivity (these elements will be hereinafter referred to as groupIIIb element) and elements belonging group Vb of the periodic tablewhich provide an n-type conductivity (these elements will be hereinafterreferred to as group Vb element).

[0100] In order for the μc-Si series thin film to contain such dopant,an appropriate raw material capable of supplying group IIIb or Vbelement is used in addition to the foregoing film-forming raw material.

[0101] Such group IIIb or Vb element-supplying raw material can includeraw materials capable of supplying group IIIb or Vb element, which arein the gaseous state at room temperature or can be easily gasified atleast under the condition for the formation of the μc-Si series thinfilm. Such group IIIb element-supplying gaseous or gasifiable rawmaterial can include boron hydrides such as B₂H₆, B₄H₁₀, B₅H₉, B₅H₁₁,B₄H₁₀, B₆H₁₂, and B₆H₁₄; and boron halides such as BF₃, BCl₃, and BBr₃.Besides, AlCl₃, GaCl₃, Ga(CH₃)₃, InCl₃, and TlCl₃ are also usable. Ofthese, B₂H₄ and BF₃ are particularly preferable.

[0102] Such group Vb element-supplying gaseous or gasifiable rawmaterial can-include phosphorous hydrides such as PH₃ and P₂H₄; andphosphorous halides such as PH₄I, PF₃, PF₅, PCl₃, PCl₅, PBr₃, PBr₅ andPI₃. Besides, AsH₃, AsF₃, AsCl₃, AsBr₃, AsF₅, SbH₃, SbF₃, SbF₅, SbCl₃,SbCl₅, BiH₃, BiCl₃, and BiBr₃ are also usable. Of these, PH₃ and PF₃ areparticularly preferable.

[0103] Any of these group IIIb or Vb elemen-supplying raw materials maybe diluted with an appropriate gas such as H₂ gas or inert gas such asAr, He, Ne, Xe or Kr, if necessary.

[0104] In the following, description will be made of experiments whichwere conducted by the present inventors during the process to accomplishthe present invention.

Experiment 1

[0105] First, description will be made of a film-forming apparatus ofthe roll-to-roll system used in this experiment.

[0106] Specifically, there was used a film-forming apparatus having suchconstitution as shown in FIG. 1. In FIG. 1, reference numeral 101indicates a vacuum chamber (or a deposition chamber) which is providedwith an exhaust pipe 107 connected to a vacuum pump (not shown) though aconductance valve 108 (comprising a butterfly valve). The exhaust pipe107 is provided with a vacuum gage 109. The inner pressure in the insideof the vacuum chamber 101 can be adjusted to a desired pressure byevacuating the inside of the vacuum chamber through the exhaust pipe byactuating the vacuum pump while properly regulating the conductancevalve 108 and observing the vacuum gage 109.

[0107] Reference numeral 102 indicates a web substrate (a longsubstrate) on which a film is to be deposited. The web substrate 102 iswound on a pay-out roll 111 provided in a load chamber 114 capable ofbeing vacuumed. The web substrate 102 is paid out from the pay-out roll111, and it passes through a gas gate 113 a which is provided at a sidewall (on the left side in the figure) of the vacuum chamber and entersinto the vacuum chamber 101. Successively, the web substrate 102 passesthrough a gas gate 113 b which is provided at a side wall (on the rightside in the figure) of the vacuum chamber and enters into an unloadchamber 115 capable of being vacuumed, where it is taken up by a take-uproll 112 provided in the unload chamber 115, followed by being wound onthe take-up roll. Here, the pay-out roll side is an upper side and thetake-up roll side is a down side in relation to the transportationdirection of the web substrate.

[0108] Specifically, by rotating the pay-out roll 111 by means of adriving motor (not shown), from the pay-out roll 111, the web substrate102 is continuously supplied, continuously moved in the vacuum chamber101 in the longitudinal direction, and wound on the take-up roll 112.That is, the web substrate 102 is transported from the left side in thefigure toward the right side in the figure.

[0109] Each of the gas gates 113 a and 113 b is structured so that theweb substrate 102 can be transported without breaking the vacuum and agate gas can be flown therein. The gas gate 113 a serves to communicatebetween the load chamber 114 and the vacuum chamber 101 whilemaintaining each of the two chambers under reduced pressure. Similarly,the gas gate 113 b serves to communicate between the vacuum chamber 101and the unload chamber 115 while maintaining each of the two chambersunder reduced pressure.

[0110] The gas gates 113 a and 113 b serve also as a communication meanswhen the vacuum chamber 101 is connected to other vacuum chamber.

[0111] Reference numeral 110 indicates gas supply pipes which are openinto the vacuum chamber 101. The gas supply pipes 110 are extended froma raw material gas supply system having a plurality of gas reservoirseach containing a given raw material gas (not shown in the figure) andthey serve to introduce desired raw material gas into the vacuum chamber101. The raw material gas introduced into the vacuum chamber 101 isexhausted through the exhaust pipe 107.

[0112] On rear side (which is not opposed to plasma 106) of the websubstrate 102 in the vacuum chamber 101, there is provided a lamp heaterunit 103 capable of radiating heat to heat the web substrate 102.Reference numeral 104 indicates a bar-like shaped electrode which iselectrically connected to a high frequency power source 105 having amatching box installed therein through a cable or a waveguide (notshown)

[0113] The formation of a μc-Si series thin film using the film-formingapparatus shown in FIG. 1 is conducted in the following manner.

[0114] The web substrate 102 from the pay-out roll 111 is passed throughthe vacuum chamber 101 and fixed to the take-up roll 112. The inside ofthe vacuum chamber 101 is evacuated to a desired vacuum through theexhaust pipe 107 by actuating the vacuum pump (not shown). Through thegas supply pipes 110, prescribed raw material gas and dilution gas areintroduced into the vacuum chamber 101. The lamp heater unit 103 isactuated, and the take-up roll 112 is rotated by means of a drivingmotor (not shown) to continuously move the web substrate 102. By this,the web substrate 102 is continuously transported while passing throughthe vacuum chamber 101, where the web substrate 102 situated in thevacuum chamber 101 is heated by the lamp heater unit 103. Then, the highfrequency power source 105 is switched on to supply a high frequencypower with a desired oscillation frequency to the bar-like shapedelectrode, where glow discharge is generated to produce plasma 106,whereby the raw material gas is decomposed by the action of the plasmato deposit a deposited film (a μc-Si series thin film) on the websubstrate which is continuously moving.

[0115] Description will be made of experiments conducted by the presentinventors.

[0116] Using the film-forming apparatus shown in FIG. 1, a μc-Si seriesthin film was formed on a web substrate 102 which is continuouslymoving, in accordance with the above-described manner.

[0117] As the web substrate 102, there was used a stainless steelSUS430BA web of 0.2 mm in thickness, 20 cm in width and 50 m in length.As the vacuum pump of the vacuum pump, there were used a rotary pump, amechanical booster pump and a turbo-molecular pump.

[0118] Specifically, the inside of each of the load chamber 114containing the pay-out roll 111, the unload chamber 115 containing thetake-up roll 112, and the vacuum chamber 101 was roughly evacuated bymeans of the rotary pump. Successively, the inside of each of the loadchamber 114 containing the pay-out roll 111, the unload chamber 115containing the take-up roll 112, and the vacuum chamber 101 wasevacuated by means of the mechanical booster pump until the pressurebecame about 1.3×10⁻¹ Pa. While maintaining the surface of the websubstrate at 300° C. by means of the lamp heater unit 103, the inside ofthe vacuum chamber 101 is evacuated to a vacuum of about 2.7×10⁻³ Pa bymeans of the turbo-molecular pump. When the inner pressure of the vacuumchamber 101 became stable at about 2.7×10⁻³ Pa, H₂ gas of 400 sccm as apurging gas from the gas supply system (not shown) was introduced intothe vacuum chamber through a mass flow controller (not shown) and thegas supply pipes 110 and a conduit (not shown) connected to the vacuumchamber 101. Successively, the inside of the vacuum chamber 101 wasevacuated by means of the turbo-molecular pump for 2 hours whileregulating the butterfly valve 108 provided at the exhaust pipe 107 sothat the reading on the vacuum gage 109 became 6.7×10⁻¹.

[0119] Thereafter, SiH₄ gas and H₂ gas from the gas supply system (notshown) were introduced into the vacuum chamber 101 through mass flowcontrollers (not shown) and the gas supply pipes 110 under conditionsdescribed below, where the butterfly valve 108 was regulated so that thereading on the vacuum gage 109 became to be in a range 1.3×10⁻¹ to1.3×10² Pa. Particularly, the above gases were firstly flown for 30minutes. Then, while the gases being continued flowing, from the highfrequency power source 105, a high frequency power with an oscillationfrequency in an range of 13.56 MHz to 1000 MHz in terms of an effectivevalue was supplied to the bar-like shaped electrode 104 to produceplasma 106, whereby a deposited film was formed on the web substrate 102over its length of 40 m. In this case, the web substrate 102 wastransported at a transportation speed of 14 cm/minute.

[0120] The film-forming conditions in this case are as follows.

[0121] Film-Forming Conditions:

[0122] raw material gas (SiH₄): 50 sccm

[0123] dilution gas (H₂): 2000 sccm

[0124] oscillation frequency: 13.56 to 1000 MHz

[0125] high frequency power: 0.05 to 50 W/cm²

[0126] inner pressure in the deposition chamber: 1.3×10⁻¹ to 1.3×10² Pa

[0127] interval between substrate and electrode: 0.5 to 30 cm

[0128] substrate temperature: 300° C.

[0129] deposited film thickness: 7 μm

[0130] deposition rate: 10 Å/sec.

[0131] electrode form: 5 cm (diameter)×25 cm (length)

[0132] After the formation of the deposited film, the web substrate 102was cooled to room temperature and taken out from the film-formingapparatus.

[0133] In this way, there were obtained a plurality of samples eachcomprising the web substrate 102 having the deposited film formedthereon (each sample will be hereinafter referred to as “web substratesample”).

[0134] For each web substrate sample, with respect to its widthdirection, part of thereof was cut to obtain an experimental sample.Thus, there were obtained a plurality of experimental samples.

[0135] For these experimental samples prepared by changing theoscillation frequency and adjusting the wattage of the high frequencypower, the inner pressure in the vacuum chamber, and the intervalbetween the substrate and the electrode so that the deposition rate wasconstant for the purpose of examining influences of the oscillationfrequency, the following evaluations were conducted. The evaluatedresults with respect to distribution of the film property in the widthdirection of the web substrate 102 are graphically shown in FIGS. 2 to5.

[0136] Evaluation Contents:

[0137] 1. Visual examination and examination by an optical microscope.

[0138] 2. Evaluation of absorption coefficient:

[0139] Wavelength dependency of the absorption coefficient of a μc-Siseries thin film was examined by measuring its transmittance using aspectrophotometer U4000 type (produced by Hitachi Ltd.).

[0140] 3. Evaluation of average grain size:

[0141] This evaluation was conducted in a manner wherein a cross sectionof the crystal is observed by means of a transmission electronmicroscope (TEM) JEM-4000EX (produced by JEOL Ltd.), respective crystalgrain boundaries are determined by way of image processing, and based onthe resultant images, an average grain size in the vicinity of thesurface and in a direction parallel to the substrate is obtained.

[0142] 4. Evaluation of crystal volume fraction:

[0143] This evaluation was conducted in a manner wherein a Ramanscattering spectrum is measured by means of a laser. Ramanspectrophotometer NRS200C (produced by Nihon Bunko Kabushiki Kaisha),followed by obtaining an intensity ratio between a strong signal from acrystal near 520 cm⁻¹ and a broad signal from an amorphous material near480 cm⁻¹, whereby obtaining a crystal volume fraction.

[0144] 5. Evaluation of hydrogen content:

[0145] This evaluation was conducted in a manner wherein an infraredabsorption spectrum is measured by means of a FTIR method and based onthe absorption near 2000 cm⁻¹, a hydrogen content is obtained.

[0146] As a result, in the case of the oscillation frequency being lessthan 50 MHz, even when film deposition was conducted at a desireddeposition rate by properly changing the wattage of the high frequencypower, the interval between the substrate and the electrode, and theinner pressure in the vacuum chamber, there was deposited an amorphoussilicon film only. As a result of observing its surface by way of visualexamination and examination by means of an optical microscope, it wasfound that the surface is white-clouded and is in a roughened state. Inaddition, there was observed the presence of polysilane in the inside ofthe vacuum chamber. In this respect, it is considered that suchpolysilane is incorporated into the film and as a result, such film isresulted.

[0147] In the case of the oscillation frequency being beyond 550 MHz,sudden disappearance of the glow discharge was firstly observed. Andeven when film deposition was conducted at a desired deposition rate byproperly changing the wattage of the high frequency power, the intervalbetween the substrate and the electrode, and the inner pressure in thevacuum chamber, there was observed unevenness (of more than 10%) for thefilm property in the width direction. In addition, the situation ofcausing such unevenness was different with respect to the longitudinaldirection of the substrate. Hence, it was found that in the case offorming a μc-Si series thin film on a long substrate, it is difficult tomaintain a desired film property for therefor.

Experiment 2

[0148] Using the same apparatus used in Experiment 1 and following theprocedures in Experiment 1, examination was conducted of how theproperties of the resulting μc-Si series thin films are changed bychanging the interval between the substrate and the electrode. Thefilm-forming conditions were made as follows.

[0149] Film-Forming Conditions:

[0150] raw material gas (SiH₄): 50 sccm

[0151] dilution gas (H₂): 2000 sccm

[0152] oscillation frequency: 200 MHz

[0153] high frequency power: 10 W/cm²

[0154] inner pressure in the deposition chamber: 2.7×10¹ Pa

[0155] interval between substrate and electrode: 0.5 to 30 cm

[0156] substrate temperature: 300° C.

[0157] deposited film thickness: 7 μm

[0158] electrode form: 5 cm (diameter)×25 cm (length)

[0159] And following the evaluation procedures in Experiment 1,evaluation was conducted.

[0160] As a result, it was found that when the interval between thesubstrate and the electrode is 0.5 cm, there is formed a deposited filmapparently of amorphous silicon. And when the interval between thesubstrate and the electrode was beyond 30 cm, the film deposition ratebecame slow, and it was substantially difficult to deposit a desiredthickness.

[0161] The results obtained in this experiment are graphically shown inFIGS. 6 to 9. As these figures illustrate, it was found that theproperty of the resulting μc-Si series thin film is varied by changingthe interval between the substrate and the electrode.

Experiment 3

[0162] Using the same apparatus used in Experiment 1 and following theprocedures in Experiment 1, examination was conducted of how theproperties of the resulting μc-Si series thin films are changeddepending on a given condition at the initial film deposition stage by amanner of changing only the interval between the substrate and theelectrode at the initial deposition stage and conducting successive filmdeposition under the same film-forming conditions. For this purpose,there was formed a pin junction type photovoltaic element. Particularly,an n-type a-Si layer, an i-type μc-Si layer and a p-type μc-Si layerwere sequentially formed on the web substrate. The formation of then-type layer and the p-type layer was conducted using the apparatusshown in FIG. 1 except for changing the high frequency power source 105to an RF power source (13.56 MHz).

[0163] The respective conditions were made as will be described below.

[0164] The Conditions at the Initial Film Deposition Stage:

[0165] raw material gas (SiH₄): 50 sccm

[0166] dilution gas (H₂): 2000 sccm

[0167] oscillation frequency: 200 MHz

[0168] high-frequency power: 50 W/cm²

[0169] inner pressure in the deposition chamber: 2.7×10¹ Pa

[0170] interval between substrate and electrode: 0.5 to 30 cm

[0171] substrate temperature: 300° C.

[0172] deposited film thickness: 300 nm

[0173] The Same Conditions Thereafter:

[0174] raw material gas (SiH₄): 50 sccm

[0175] dilution gas (H₂): 2000 sccm

[0176] oscillation frequency: 200 MHz

[0177] high frequency power: 50 W/cm²

[0178] inner pressure in the deposition chamber: 2.7×10¹ Pa

[0179] interval between substrate and electrode: 5 cm

[0180] substrate temperature: 300° C.

[0181] deposited film thickness: 6.7 μm

[0182] electrode form: 5 cm (diameter)×25 cm (length)

[0183] The Conditions for the Formation of the n-type Layer:

[0184] SiH₄: 5 sccm

[0185] H₂: 50 sccm

[0186] PH₃/H₂(5%): 0.1 sccm

[0187] RF power: 0.1 W/cm²

[0188] inner pressure in the deposition chamber: 1.1×10² Pa

[0189] substrate temperature: 300° C.

[0190] deposited film thickness: 30 nm

[0191] The Conditions for the Formation of the p-type Layer:

[0192] SiH₄: 5 sccm

[0193] H₂: 200 sccm

[0194] BF₃/H₂ (5%): 0.1 sccm

[0195] RF power: 3 W/cm²

[0196] inner pressure in the deposition chamber: 2.0×10² Pa

[0197] substrate temperature: 200° C.

[0198] deposited film thickness: 20 nm

[0199] For the photovoltaic element thus formed, its crystal crosssection was observed by means of a transmission electron microscope(TEM) JEM-4000EX (produced by JEOL Ltd.), and the thickness of a portionconsidered to comprise an a-Si from an initially deposited portion wasevaluated. The results obtained are graphically shown in FIG. 10.Further, for the photovoltaic element, evaluation of its initialphotoelectric conversion efficiency and light degradation test wereconducted. The results obtained are collectively shown in Table 1.

[0200] The evaluation of initial photoelectric conversion efficiency wasconducted by placing a solar cell prepared using the photovoltaicelement under the irradiation of pseudo sunlight of AM-1.5 (100 mW/cm²)and measuring its V-I characteristics.

[0201] The light degradation test was conducted by placing the solarcell having been subjected Lo the evaluation of the initialphotoelectric conversion efficiency in the above in an environment witha humidity of 50% and a temperature of 25° C. while irradiating pseudosunlight of AM-1.5 thereto for 500 hours, measuring its photoelectricconversion efficiency under the irradiation of pseudo sunlight of AM-1.5(100 mW/cm²) in the same manner as in the above and calculating adeteriorated rate of the initial photoelectric conversion efficiency.

[0202] Based on the results shown in Table 1, there were obtained suchfindings as will be described in the following. That is, by changing theinterval between the substrate and the electrode, it is possible toprevent the deposition of an amorphous layer at the initial filmdeposition stage and a layer whose constituent crystal grains being ofsmall size. And the presence of these layers deteriorates thecharacteristics of a semiconductor device such as a photovoltaicelement.

[0203] In the following, the present invention will be described in moredetail with reference to examples. It should be understood that theseexamples are only for the illustrative purposes and are not intended torestrict the scope of the present invention.

EXAMPLE 1

[0204] As the bar-like shaped electrode 104 in the film-formingapparatus shown in FIG. 1, as shown in FIG. 11 (which is a conceptualdiagram which is not always in agreement with actual experimentalconditions), a plurality of bar-like shaped electrodes 204 each beingthe same as the bar-like shaped electrode 104 used in Experiment 1 werearranged such that they are perpendicular to a normal line of a longsubstrate 102 and their intervals to the long substrate are partiallydiffered.

[0205] Using the apparatus thus constituted, a μc-Si series thin filmwas formed under such conditions as will be described below.

[0206] As the long substrate 102, there was used a stainless steelSUS430BA web of 0.2 mm in thickness, 20 cm in width and 50 m in lengthhaving a 1 μm thick zinc oxide (ZnO) film formed thereon. The formationof μc-Si series thin film was conducted in accordance with theprocedures in Experiment 1, except for changing the film-formingconditions in Experiment 1 to those below mentioned.

[0207] Film-Forming Conditions:

[0208] raw material gas (SiH₄): 50 sccm

[0209] dilution gas (H₂): 2000 sccm

[0210] oscillation frequency: 200 MHz

[0211] high frequency power: 10 W/cm²

[0212] inner pressure in the deposition chamber: 2.7×10⁻¹ Pa

[0213] intervals of the electrodes to the long substrate (from the upperside of the substrate transportation direction):

[0214] 3 cm interval: 10 bar-like shaped electrodes,

[0215] 5 cm interval: 10 bar-like shaped electrodes,

[0216] 10 cm interval: 10 bar-like shaped electrodes,

[0217] 5 cm interval: 10 bar-like shaped electrodes, and

[0218] 4 cm interval: 10 bar-like shaped electrodes

[0219] interval between each adjacent electrodes (the distance from thecenter on the central axis of one bar-like shaped electrode to that ofthe other bar-like shaped electrode):

[0220] 10 cm

[0221] substrate temperature: 300° C.

[0222] deposited film thickness: 10 μm

COMPARATIVE EXAMPLE 1-1

[0223] Using a flat plate type electrode 1004 as shown in FIG. 12 (whichis a conceptual diagram which is not always in agreement with actualexperimental conditions), the formation of μc-Si series thin film wasconducted under film-forming conditions similar to those in Example 1.

[0224] The flat plate type electrode 1004 herein is of a length of 5 m.The interval between the substrate and the electrode was graded to befrom 3 cm to 5 cm from the upper side of the substrate transportationdirection. Here, reference numeral 1007 in FIG. 12 schematically shows aplasma high density region.

COMPARATIVE EXAMPLE 1-2

[0225] Using a plurality of bar-like shaped electrode 104 as shown inFIG. 13 (which is a conceptual diagram which is not always in agreementwith actual experimental conditions), the formation of μc-Si series thinfilm was conducted under film-forming conditions similar to those inExample 1.

[0226] In this example, 50 bar-like shaped electrodes were spacedlyarranged so as to have an equal interval of 3 cm to a long substrate102. And the interval between each adjacent electrodes (the distance ina horizontal direction from the center on the central axis of onebar-like shaped electrode to that of the other bar-like shapedelectrode) was made to be 10 cm.

[0227] In each of Example 1 and Comparative Examples 1-1 and 1-2, plasmadischarge was maintained for 10 hours, where the state of plasmaproduced was observed by way of visual examination. As a result, in eachof Example 1 and Comparative Example 1-2, the state of the plasma wasstable all the time. But in Comparative Example 1-1, such a localizedplasma region that is schematically shown as a plasma high densityregion 1007 in FIG. 12 or a partially dense plasma region was caused. Inaddition, there was observed a phenomenon in that the dischargingposition was suddenly changed during the plasma discharge.

[0228] As a result of having measured a film thickness distribution tothe prescribed 10 μm, the film thickness distribution in each of Example1 and Comparative Example 1-2 was found to fall within 15% for both thewidth direction and the longitudinal direction of the substrate. On theother hand, in Comparative Example 1-1, it was found to be beyond 15%.

[0229] As a result of having evaluated a distribution with respect toeach of absorption coefficient, crystal volume fraction, and hydrogencontent, the distribution in each of Example 1 and Comparative Example1-2 was found to fall within 15% with respect to the width direction andthe longitudinal direction of the substrate. On the other hand, inComparative Example 1-1, amorphous portions were present, and thedistribution was found to be beyond 15%.

[0230] In each of Example 1 and Comparative Example 1-2, evaluation wasconducted with respect to profile of each of absorption coefficient,crystal volume fraction, and hydrogen content in depth direction in thefollowing manner.

[0231] Evaluation of Absorption Coefficient:

[0232] In order to examine a variation in a band profile of absorptioncoefficient for a thin film sample, a variation in the absorptioncoefficient in the depth direction is evaluated in a manner of polishinga predetermined thickness of the thin film sample by means of CMP(chemical-mechanical polishing), and measuring the transmittance thereofby means of a spectrophotometer U4000 type (produced by Hitachi Ltd.).The absorption coefficient corresponds an average value thereof when theabsorption coefficient is considered to be distributed with the film.Therefore, it is possible to estimate a distribution of the absorptioncoefficient by observing how the absorption coefficient is changed alongwith a change in the film thickness.

[0233] Evaluation of Average Grain Size:

[0234] This evaluation is conducted in a manner wherein a crystal crosssection of a thin film sample is observed by means of a transmissionelectron microscope (TEM) JEM-4000EX (produced by JEOL Ltd.), respectivecrystal grain boundaries are determined by way of image processing, andbased on the resultant images, a variation in an average grain size inthe depth direction in a direction parallel to the substrate isexamined.

[0235] Evaluation of Crystal Deposition Rate:

[0236] This evaluation is conducted as follows. That is, for a thin filmsample (which is formed on the ZnO (of polycrystal) of the substrate),when the substrate is subjected to plastic deformation at an acute angleof more than 150°, there is afforded a cross section of the μc-Si layertogether with a wall interface of the ZnO polycrystal. For said crosssection, by using a laser Raman spectrophotometer NRS200C (produced byNihon Bunko Kabushiki Kaisha) while throttling the spot of the laserbeam, a Raman scattering spectrum is measured with a resolving power of1 μm, followed by obtaining an intensity ratio between a strong signalfrom a crystal near 520 cm⁻¹ and a broad signal from an amorphousmaterial near 480 cm⁻¹, and based on the intensity ratio, a variation inthe crystal deposition rate in the depth direction is evaluated.

[0237] Evaluation of Hydrogen Content:

[0238] This evaluation is conducted by subjecting a thin film sample tohydrogen content analysis by way of SIMS (secondary ion massspectrometry) to obtain a hydrogen content profile in the depthdirection.

[0239] In the following, description will be made of the evaluatedresults.

[0240] With Respect to the Absorption Coefficient:

[0241] The absorption coefficient in Comparative Example 1-2 was foundto be simply decreased. Particularly, the absorption coefficient of aportion (which is about 200 nm thick) corresponding to the initialdeposition stage is small, and for the absorption thereafter, it wellcoincides with a model in which substantially constant absorptioncoefficient is assumed.

[0242] On the other hand, the absorption coefficient in Example 1 wasfound to be increased and decreased. Particularly, it was found that alayer having a large absorption coefficient and a layer having a smallabsorption coefficient are stacked in the thickness direction.

[0243] With Respect to the Average Grain Size:

[0244] The evaluated results of this evaluation item are shown Table 2.

[0245] In Comparative Example 1-2, there was obtained a finding thatalthough an amorphous layer is present and the average grain size isgradually increased until near 200 nm from the substrate, the averagegrain size thereafter is substantially constant until near the surface.

[0246] In Example 1, there was obtained a finding that the state at theinitial deposition stage in Example 1 is around the same as that inComparative Example 1-2, but the average grain size thereafter is variedsuch that it is increased and decreased in the depth direction.

[0247] In FIG. 14(a), there is shown a part of the cross section by TEMin Comparative Example 1-2. In FIG. 14(b), there is shown a part of thecross section by TEM in Example 1. It is understood that portion wheresubstantially no grain size-changed portion is present in the crosssection of Comparative Example 1-2 as shown in FIG. 14(a), but in thecross section of Example 1, as shown in FIG. 14(b), there are presentgrain size-changed portions A-A′ and B-B′.

[0248] With Respect to the Crystal Volume Fraction:

[0249] The evaluated results of this evaluation item are shown in Table3. Based on the results shown in Table 3, the crystal volume fraction inComparative Example 1-2 was found to be substantially constant untilnear the surface. On the other hand, the crystal volume fraction inExample 1 was found to be varied such that it is first increased andthen decreased.

[0250] With Respect to the Hydrogen Content:

[0251] The evaluated results of this evaluation item are graphicallyshown in FIG. 15. Based on the results shown in FIG. 15, the followingfacts are understood. That is, in Comparative Example 1-2, a layerhaving a large hydrogen content is present until near 200 nm from thesubstrate, and although the hydrogen content thereafter is decreased,the successive hydrogen content is substantially constant until near thesurface. On the other hand, in Example 1, although the state at theinitial deposition stage is around the same as that in ComparativeExample 1-2, the hydrogen content thereafter is varied such that it isincreased and then decreased.

[0252] As will be understood from the above description, it isunderstood that according to the method of Example 1, it is possible toreadily form a μc-Si series thin film whose property being desirablycontrolled in the thickness direction.

EXAMPLE 2

[0253] As the bar-like shaped electrode 104 in the film-formingapparatus shown in FIG. 1, as shown in FIG. 16 (which is a conceptualdiagram which is not always in agreement with actual experimentalconditions), a plurality of bar-like shaped electrodes 104 each beingthe same as the bar-like shaped electrode 104 used in Experiment 1 werearranged in parallel to each other such that they are perpendicular to anormal line of a long substrate 102 and their intervals to the longsubstrate are partially differed.

[0254] Using the apparatus thus constituted, a μc-Si series thin filmwas formed under such conditions as will be described below.

[0255] As the long substrate 102, there was used a stainless steelSUS430BA web of 0.2 mm in thickness, 20 cm in width and 50 m in lengthhaving a 1 μm thick zinc oxide (ZnO) film formed thereon. The formationof μc-Si series thin film was conducted in accordance with theprocedures in Experiment 1, except for changing the film-formingconditions in Experiment 1 to those below mentioned.

[0256] Film-Forming Conditions:

[0257] raw material gas (SiH₄): 50 sccm

[0258] dilution gas (H₂): 2000 sccm

[0259] oscillation frequency: 200 MHz

[0260] high frequency power: 10 W/cm²

[0261] inner pressure in the deposition chamber: 2.7×10⁻¹ Pa

[0262] intervals of the electrodes to the long substrate (from the upperside of the substrate transportation direction):

[0263] 3 cm interval: 10 bar-like shaped electrodes,

[0264] 5 cm interval: 10 bar-like shaped electrodes,

[0265] 5 cm interval: 10 bar-like shaped electrodes,

[0266] 5 cm interval: 10 bar-like shaped electrodes, and

[0267] 5 cm interval: 10 bar-like shaped electrodes

[0268] interval between each adjacent electrodes (the distance in ahorizontal direction from the center on the central axis of one bar-likeshaped electrode to that of the other bar-like shaped electrode):

[0269] 10 cm

[0270] temperature: 300° C.

[0271] film thickness: 10 μm

EXAMPLE 3

[0272] The procedures of Example 2 were repeated, except that thearrangement of the bar-like shaped electrodes was changed to be not inparallel to each other, to form a μc-Si series thin film.

Evaluation

[0273] Evaluation with respect to film thickness distribution in thewidth direction of the substrate was conducted in the same manner as inExample 1. As a result, the film thickness distribution in Example 2 wasfound to fall within 10%, but that in Example 3 was found to be 13%.

[0274] As a result of having evaluated a distribution with respect toeach of absorption coefficient, crystal deposition rate, and hydrogencontent respectively in the width direction of the substrate, thedistribution in Example 2 was found to fall within 10%, but that inExample 3 was found to be 13%.

[0275] Based on the above results, it is understood that by arrangingthe bar-like shaped electrodes in parallel to each other as in Example2, it is possible to diminish unevenness in the film thickness in thewidth direction of the substrate.

EXAMPLE 4

[0276] As the bar-like shaped electrode 104 in the film-formingapparatus shown in FIG. 1, as shown in FIG. 17 (which is a conceptualdiagram which is not always in agreement with actual experimentalconditions), a plurality of bar-like shaped electrodes 104 each beingthe same as the bar-like shaped electrode 104 used in Experiment 1 werearranged in parallel to each other such that they are perpendicular to anormal line of a long substrate 102, they are perpendicular to thedirection for the long substrate to be transported, and their intervalsto the long substrate are partially differed.

[0277] Using the apparatus thus constituted, a μc-Si series thin filmwas formed under such conditions as will be described below.

[0278] As the long substrate 102, there was used a stainless steelSUS430BA web of 0.2 mm in thickness, 20 cm in width and 50 m in lengthhaving a 1 μm thick zinc oxide (ZnO) film formed thereon. The formationof μc-Si series thin film was conducted in accordance with theprocedures in Experiment 1, except for changing the film-formingconditions in Experiment 1 to those below mentioned.

[0279] Film-Forming Conditions:

[0280] raw material gas (SiH₄): 50 sccm

[0281] dilution gas (H₂): 2000 sccm

[0282] oscillation frequency: 200 MHz

[0283] high frequency power: 10 W/cm²

[0284] inner pressure in the deposition chamber: 2.7×10⁻¹ Pa

[0285] intervals of the electrodes to the long substrate (from the upperside of the substrate transportation direction):

[0286] 3 cm interval: 10 bar-like shaped electrodes,

[0287] 5 cm interval: 10 bar-like shaped electrodes,

[0288] 5 cm interval: 10 bar-like shaped electrodes,

[0289] 5 cm interval: 10 bar-like shaped electrodes, and

[0290] 5 cm interval: 10 bar-like shaped electrodes

[0291] angle of each electrode to the direction for the substrate to betransported: 90°

[0292] interval between each adjacent electrodes (the distance in ahorizontal direction from the center on the central axis of one bar-likeshaped electrode to that of the other bar-like shaped electrode):

[0293] 10 cm

[0294] substrate temperature: 300° C.

[0295] deposited film thickness: 10 μm

Evaluation

[0296] Evaluation with respect to film thickness distribution in thewidth direction of the substrate was conducted in the same manner as inExample 1. As a result, the film thickness distribution in Example 4 wasfound to fall within 8%, which is superior to Example 2 in terms ofuniformity.

[0297] And, as a result of having evaluated a distribution with respectto each of absorption coefficient, crystal deposition rate, and hydrogencontent respectively in the width direction of the substrate, thedistribution in Example 4 was found to fall within 8%, which is superiorto Example 2 in terms of uniformity.

[0298] Based on the above results, it is understood that by arrangingthe bar-like shaped electrodes to be perpendicular to the transportationdirection of the substrate as above described, it is possible to morediminish unevenness in the film thickness in the width direction of thesubstrate.

EXAMPLE 5

[0299] As the bar-like shaped electrode 104 in the film-formingapparatus shown in FIG. 1, as shown in FIG. 18 (which is a conceptualdiagram which is not always in agreement with actual experimentalconditions), a plurality of bar-like shaped electrodes 104 each beingthe same as the bar-like shaped electrode 104 used in Experiment 1 werearranged such that they are perpendicular to a normal line of a longsubstrate 102 and their intervals to the long substrate are widened inthe upper side of the transportation direction of the long substrate andnarrowed in the down side thereof.

[0300] Using the apparatus thus constituted, a μc-Si series thin filmwas formed under such conditions as will be described below.

[0301] As the long substrate 102, there was used a stainless steelSUS430BA web of 0.2 mm in thickness, 20 cm in width and 50 m in lengthhaving a 1 μm thick zinc oxide (ZnO) film formed thereon. The formationof μc-Si series thin film was conducted in accordance with theprocedures in Experiment 1, except for changing the film-formingconditions in Experiment 1 to those below mentioned.

[0302] Film-Forming Conditions:

[0303] raw material gas (SiH₄): 50 sccm

[0304] dilution gas (H₂): 2000 sccm

[0305] oscillation frequency: 200 MHz

[0306] high frequency power: 10 W/cm²

[0307] inner pressure in the deposition chamber: 2.7×10⁻¹ Pa

[0308] intervals of the electrodes to the long substrate (from the upperside of the substrate transportation direction):

[0309] 15 cm interval: 5 bar-like shaped electrodes,

[0310] 12 cm interval: 5 bar-like shaped electrodes,

[0311] 10 cm interval: 5 bar-like shaped electrodes,

[0312] 8 cm interval: 10 bar-like shaped electrodes,

[0313] 6 cm interval: 10 bar-like shaped electrodes, and

[0314] 4 cm interval: 15 bar-like shaped electrodes

[0315] angle of each electrode to the transportation direction of thesubstrate: 90°

[0316] interval between each adjacent electrodes (the distance in ahorizontal direction from the center on the central axis of one bar-likeshaped electrode to that of the other bar-like shaped electrode):

[0317] 10 cm

[0318] substrate temperature: 300° C.

[0319] deposited film thickness: 10 μm

COMPARATIVE EXAMPLE 2

[0320] The procedures of Example 5 were repeated, except that all theintervals of the 50 bar-like shaped electrodes to the long substratewere made to be 4 cm, to form a μc-Si series thin film.

Evaluation

[0321] For each of Example 5 and Comparative Example 2, evaluation wasconducted with respect to profile of each of absorption coefficient,crystal volume fraction, and hydrogen content in depth direction in thesame manner as in Example 1.

[0322] In the following, description will be made of the evaluatedresults.

[0323] With Respect to the Absorption Coefficient:

[0324] The absorption coefficient in Comparative Example 2 was found tobe simply decreased. Particularly, the absorption coefficient of aportion (which is about 200 nm thick) corresponding to the initialdeposition stage is small, and for the absorption thereafter, it wellcoincides with a model in which substantially constant absorptioncoefficient is assumed.

[0325] On the other hand, the absorption coefficient in Example 5 wasfound to be gradually increased. Specifically, it was found that theabsorption coefficient is gradually enlarged as the film depositionproceeds.

[0326] With Respect to the Average Grain Size:

[0327] The evaluated results of this evaluation item are shown Table 4.

[0328] The following facts were found. That is, in Comparative Example2, although an amorphous layer is present and the average grain size isgradually increased until near 200 nm from the substrate, the averagegrain size thereafter is substantially constant until near the surface.

[0329] On the other hand, in Example 5, substantially no amorphous layeris present in the state at the initial deposition stage, and the averagegrain size thereafter is gradually decreased toward near the surface.

[0330] With Respect to the Crystal Volume Fraction:

[0331] The evaluated results of this evaluation item are shown in Table5. Based on the results shown in Table 5, it is understood that thecrystal volume fraction in Comparative Example 2 is substantiallyconstant until near the surface, but the crystal volume fraction inExample5 is gradually decreased toward near the surface.

[0332] With Respect to the Hydrogen Content:

[0333] The evaluated results of this evaluation item are graphicallyshown in FIG. 19. Based on the results shown in FIG. 19, the followingfacts are understood. That is, in Comparative Example 2, a layer havinga large hydrogen content is present until near 200 nm from thesubstrate, and although the hydrogen content thereafter is decreased,the successive hydrogen content is substantially constant until near thesurface. On the other hand, in Example 5, although the hydrogen contentin the state at the initial deposition stage is substantially constant,the hydrogen content thereafter is gradually increased toward near thesurface.

[0334] Separately, in each of Example 5 and Comparative Example 2, therewas prepared a photovoltaic element as well as in Experiment 3, whereinthe formation of the p-type layer and the n-type layer was conductedunder the same conditions employed for the formation of these layers inExperiment 3. For the resultant photovoltaic elements, evaluation withrespect to initial photoelectric conversion efficiency and lightdegradation test were conducted in the same manner as in Experiment 3.

[0335] The results obtained are collectively shown in Table 6. Eachvalue for Example 5 is a value relative to the corresponding value ofComparative Example 2 which is set at 1.00.

[0336] Based on the results shown in Table 6, it is understood that byusing a μc-Si series thin film formed by using a plurality of bar-likeshaped electrodes arranged such that their intervals to a long substrateare widened in the upper side of the transportation direction of thelong substrate and narrowed in the down side thereof, it is possible toprepare a photovoltaic element which is superior in photovoltaic elementcharacteristics.

EXAMPLE 6

[0337] As the bar-like shaped electrode 104 in the film-formingapparatus shown in FIG. 1, as shown in FIG. 20 (which is a conceptualdiagram which is not always in agreement with actual experimentalconditions), a plurality of bar-like shaped electrodes 104 each beingthe same as the bar-like shaped electrode 104 used in Experiment 1 werearranged such that they are perpendicular to a normal line of a longsubstrate 102 and their intervals to the long substrate are partiallyperiodically changed to the transportation direction of the longsubstrate.

[0338] Using the apparatus thus constituted, a μc-Si series thin filmwas formed under such conditions as will be described below.

[0339] As the long substrate 102, there was used a stainless steelSUS430BA web of 0.2 mm in thickness, 20 cm in width and 50 m in lengthhaving a 1 μm thick zinc oxide (ZnO) film formed thereon. The formationof μc-Si series thin film was conducted in accordance with theprocedures in Experiment 1, except for changing the film-formingconditions in Experiment 1 to those below mentioned.

[0340] Film-Forming Conditions:

[0341] raw material gas (SiH₄): 50 sccm

[0342] dilution gas (H₂): 2000 sccm

[0343] oscillation frequency: 200 MHz

[0344] high frequency power: 10 W/cm²

[0345] inner pressure in the deposition chamber: 2.7×10⁻¹ Pa

[0346] intervals of the electrodes to the long substrate (from the upperside of the substrate transportation direction):

[0347] 15 cm interval: 5 bar-like shaped electrodes,

[0348] 12 cm interval: 5 bar-like shaped electrodes,

[0349] 4 cm interval: 10 bar-like shaped electrodes,

[0350] 8 cm interval: 10 bar-like shaped electrodes,

[0351] 4 cm interval: 10 bar-like shaped electrodes, and

[0352] 8 cm interval: 10 bar-like shaped electrodes

[0353] angle of each electrode to the transportation direction of thesubstrate: 90°

[0354] interval between each adjacent electrodes (the distance in ahorizontal direction from the center on the central axis of one bar-likeshaped electrode to that of the other bar-like shaped electrode):

[0355] 10 cm

[0356] substrate temperature: 300° C.

[0357] deposited film thickness: 10 μm

Evaluation

[0358] For Example 6, evaluation was conducted with respect to profileof each of absorption coefficient, crystal volume fraction, and hydrogencontent in depth direction in the same manner as in Example 1.

[0359] In the following, description will be made of the evaluatedresults.

[0360] With Respect to the Absorption Coefficient:

[0361] The absorption coefficient in Example 6 was found to beperiodically changed such that its increase and decrease are repeated.Specifically, it was found that the absorption coefficient isperiodically changed as the film deposition proceeds.

[0362] With Respect to the Average Grain Size:

[0363] The evaluated results of this evaluation item are shown Table 7.

[0364] In Example 6, substantially no amorphous layer was present in thestate at the initial deposition stage, and the average grain sizethereafter was periodically changed such that it was increased anddecreased toward near the surface.

[0365] With Respect to the Crystal Volume Fraction:

[0366] The evaluated results of this evaluation item are shown in Table8. Based on the results shown in Table 8, it is understood that thecrystal volume fraction in Example 6 is periodically changed such thatit is increased and decreased toward near the surface.

[0367] With Respect to the Hydrogen Content:

[0368] The evaluated results of this evaluation item in Example 6 aregraphically shown in FIG. 21, in which the evaluated results of thisevaluation item obtained in Comparative Example 2 are together shown.Based on the results shown in FIG. 21, the following facts areunderstood. That is, in Example 6, although the hydrogen content in thestate at the initial deposition stage is substantially constant, thehydrogen content thereafter is periodically changed such that it isincreased and decreased toward near the surface.

[0369] From the above results, it is understood that according to theprocess of Example 6, it is possible to readily form a μc-Si series thinfilm whose absorption coefficient is periodically changed toward thefilm thickness direction.

[0370] Separately, in Example 6, there was prepared a photovoltaicelement as well as in Experiment 3, wherein the formation of the p-typelayer and the n-type layer was conducted under the same conditionsemployed for the formation of these layers in Experiment 3. For theresultant photovoltaic element, evaluation with respect to initialphotoelectric conversion efficiency and light degradation test wereconducted in the same manner as in Experiment 3.

[0371] The results obtained are collectively shown in Table 9, in whichthe evaluated results of the photovoltaic element in Comparative Example2 are together shown. Each value for Example 6 is a value relative tothe corresponding value of Comparative Example 2 which is set at 1.00.

[0372] Based on the results shown in Table 9, it is understood that byusing a μc-Si series thin film formed by using a plurality of bar-likeshaped electrodes arranged such that their intervals to a long substrateare periodically changed to the transportation direction of the longsubstrate, it is possible to prepare a photovoltaic element which issuperior in photovoltaic element characteristics.

[0373] As will be understood from the above description, according tothe process of the present invention, it is possible to readily controlthe properties of a μc-Si series thin film in the film thicknessdirection by a simple manner. And it is possible to readily form a highquality μc-Si series thin film excelling in properties and which has agraded film property in the film thickness direction. In addition, byusing such μc-Si series thin film, it is possible to produce a highquality microcrystal semiconductor device at a reasonable productioncost. TABLE 1 interval between substrate initial photoelectric andelectrode (cm) conversion efficiency degradation test 0.5 0.80 0.85 0.80.90 0.95 1.0 0.95 0.97 2.0 0.97 0.99 5.0 1.00 1.00 8.0 1.05 1.03 10  1.05 1.03 20   1.05 1.03

[0374] TABLE 2 depth direction (from substrate) (μm) average grain size1 3 5 7 9 Example 1 1.00 1.10 1.20 1.10 1.05 Comparative Example 1-21.00 1.00 1.00 1.00 1.00

[0375] TABLE 3 depth direction (from substrate) (μm) crystal volumefraction 1 3 5 7 9 Example 1 1.00 1.08 1.20 1.10 1.6  ComparativeExample 1-2 1.00 1.00 1.00 1.00 1.00

[0376] TABLE 4 depth direction (from substrate) (μm) average grain size1 3 5 7 9 Example 5 1.00 0.96 0.92 0.90 0.84 Comparative Example 2 1.001.00 1.00 1.00 1.00

[0377] TABLE 5 depth direction (from substrate) (μm) crystal volumefraction 1 3 5 7 9 Example 5 1.00 0.97 0.93 0.89 0.83 ComparativeExample 2 1.00 1.00 1.00 1.00 1.00

[0378] TABLE 6 initial photoelectric conversion efficiency degradationtest Example 5 1.12 1.09 Comparative Example 2 1.00 1.00

[0379] TABLE 7 depth direction (from substrate) (μm) average grain size1 3 5 7 9 Example 6 1.00 0.80 0.93 0.82 0.94

[0380] TABLE 8 depth direction (from substrate) (μm) crystal volumefraction 1 3 5 7 9 Example 6 1.00 0.84 0.90 0.79 0.92

[0381] TABLE 9 initial photoelectric conversion efficiency degradationtest Example 6 1.05 1.11 Comparative Example 2 1.00 1.00

What is claimed is:
 1. A process for forming a microcrystalline siliconseries thin film by arranging a long substrate in a vacuum chamber so asto oppose an electrode provided in said vacuum chamber and whiletransporting said long substrate in a longitudinal direction, causingglow discharge between said electrode and said long substrate to depositsaid microcrystalline silicon series thin film on said long substrate,wherein a plurality of bar-like shaped electrodes as said electrode arearranged such that they are perpendicular to a normal line of said longsubstrate and their intervals to said long substrate are all orpartially differed, and said glow discharge is caused using a highfrequency power with an oscillation frequency in a range of from 50 MHzto 550 MHz, whereby depositing said microcrystalline series thin film onsaid long substrate.
 2. A process according to claim 1 , wherein saidplurality of bar-like shaped electrodes are arranged such that they arein parallel to each other.
 3. A process according to claim 1 , whereinsaid plurality of bar-like shaped electrodes are arranged such that theyare perpendicular to a transportation direction of the long substrate.4. A process according to claim 1 , wherein said plurality of bar-likeshaped electrodes are arranged such that their intervals to the longsubstrate are widened in an upper side of a transportation direction ofthe long substrate and narrowed in a down side thereof.
 5. A processaccording to claim 1 , wherein said plurality of bar-like shapedelectrodes are arranged such that their intervals to the long substrateare periodically changed to a transportation direction of the longsubstrate.
 6. An apparatus for forming a microcrystalline silicon seriesthin film on a long substrate, having a portion in which said longsubstrate is arranged to oppose to an electrode in a vacuum chamber,wherein while transporting said long substrate in a longitudinaldirection, glow discharge is caused between said electrode and said longsubstrate to deposit said microcrystalline series thin film on said longsubstrate, wherein said apparatus has a plurality of bar-like shapedelectrodes as said electrode which are arranged such that they areperpendicular to a normal line of said long substrate and theirintervals to said long substrate are all or partially differed and ahigh frequency power source for causing said glow discharge using a highfrequency power with an oscillation frequency in a range of from 50 MHzto 550 MHz.
 7. An apparatus according to claim 6 , wherein saidplurality of bar-like shaped electrodes are arranged such that they arein parallel to each other.
 8. An apparatus according to claim 6 ,wherein said plurality of bar-like shaped electrodes are arranged suchthat they are perpendicular to a transportation direction of the longsubstrate.
 9. An apparatus according to claim 6 , wherein said pluralityof bar-like shaped electrodes are arranged such that their intervals tothe long substrate are widened in an upper side of a transportationdirection of the long substrate and narrowed in a down side thereof. 10.An apparatus according to claim 6 , wherein said plurality of bar-likeshaped electrodes are arranged such that their intervals to the longsubstrate are periodically changed to a transportation direction of thelong substrate.